This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application No. 2000-351661 filed on Nov. 17, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to a vehicle brake system. More particularly, the present invention pertains to a hydraulic brake device used in a vehicle brake system.
A known hydraulic brake device is disclosed in U.S. Pat. No. 3,559,406. This hydraulic brake device includes a large diameter piston movable by a power piston of a vacuum booster actuated to generate boosting force by utilizing a vacuum source of the engine upon braking operation. A small diameter piston is fitted into the inner periphery of the large diameter piston to be movable relative to the large diameter piston upon functional failure of the vacuum booster.
In operation, when the large diameter piston receives the boosting force from the vacuum booster, a sufficient stroke for supplying brake pressure to a wheel brake cylinder is needed and a relatively large axial length is necessary for the piston to achieve braking operation considering the positional relationship between the cylinder portion and the sealing member disposed for sealing the cylinder portion. Further, in the event the vacuum booster fails, the small diameter piston has to move within the inner periphery of the large diameter piston for a sufficient stroke to effectively achieve the braking operation. Thus, the axial length of the large diameter piston has to have a sufficient length, and this undesirably lengthens the entire piston.
Thus, a need exists for a hydraulic brake device having a piston of reduced length as compared to the known hydraulic brake device described above.
A need also exists for a hydraulic brake device which possesses a shortened piston, but which nevertheless can be manufactured relatively easily and at a relatively low cost.
According to one aspect of the present invention, a hydraulic brake device includes a large diameter piston movable within a cylinder in response to a operation of a power piston of a booster, a small diameter piston engageable with an inner periphery of the larger diameter piston and movable relative to the large diameter piston in response to the brake operation, a floating piston engageable with the small diameter piston and movable relative to the small diameter piston, a first pressure chamber defined by the large diameter piston, the small diameter piston and the floating piston, a second pressure chamber defined by the cylinder and the floating piston, and a relief chamber defined by the cylinder and the large diameter piston. A switching device is disposed between the large diameter piston and the small diameter piston to control fluid communication between the first pressure chamber and the relief chamber to normally interrupt the fluid communication between the first pressure chamber and the relief chamber and to establish fluid communication between the first pressure chamber and the relief chamber upon relative movement between the large diameter piston and the small diameter piston.
During braking operation, when the booster is actuated to boost the brake force, the floating piston moves relative to the small diameter piston in the first pressure chamber formed by the large and small diameter pistons and the floating piston. In the event the booster function fails, both the floating piston and the small diameter piston move integrally in the first pressure chamber. This advantageously permits the axial length of the large diameter piston or small diameter piston to be shortened compared to the pistons used in other known devices, thus facilitating the manufacturing process and reduce the cost.
According to another aspect of the invention, a hydraulic brake device includes a cylinder, a larger diameter piston movable within the cylinder in response to movement of a power piston of a brake booster, a smaller diameter piston engaged with an inner peripheral portion of the large diameter piston and movable relative to the larger diameter piston in response to operation of a brake operation member, a floating piston engaged with the smaller diameter piston and movable relative to the smaller diameter piston, a first pressure chamber defined by the larger diameter piston, the smaller diameter piston and the floating piston, a second pressure chamber defined by the cylinder and the floating piston and hydraulically connected to a wheel brake cylinder, and a relief chamber defined by the cylinder and the larger diameter piston. A seal member is provided on one of the larger diameter piston and the smaller diameter piston for interrupting fluid communication between the first pressure chamber and the relief chamber, and a port is provided on the other of the larger diameter piston and the smaller diameter piston, with fluid communication between the first pressure chamber and the relief chamber being established by way of the port upon relative movement between the larger diameter piston and the smaller diameter piston.
In accordance with another aspect of the invention, a hydraulic brake system for a vehicle includes a brake operation member operated by a driver of the vehicle to generate a brake operation force, a booster device having a power piston operatively connected to the brake operation member for boosting the brake operation force generated by the brake operation member, a wheel brake cylinder for producing braking force in response to the brake operation force, and a brake device. The brake device includes a cylinder, a larger diameter piston movable within the cylinder in response to movement of the power piston of the booster, a smaller diameter piston engaged with an inner peripheral portion of the larger diameter piston and movable relative to the larger diameter piston in response to the operation of the brake operation member, a floating piston engaged with the smaller diameter piston and movable relative to the smaller diameter piston, a first pressure chamber defined by the larger diameter piston and the smaller diameter piston, a second pressure chamber defined by the cylinder and the floating piston and hydraulically connected to the wheel brake cylinder, a relief chamber defined by the cylinder and the larger diameter piston, and a switching device disposed between the larger diameter piston and the smaller diameter piston. The switching device normally interrupts fluid communication between the first pressure chamber and the relief chamber and establishes fluid communication between the first pressure chamber and the relief chamber upon relative movement between the larger diameter piston and the smaller diameter piston.